Altitude Critical AreasRobert Sumwalt, long-time accident investigator and aviation safety author and now vice chairman of the National Transportation Safety Board (NTSB), coined a term some time back he called the Altitude Critical Area (ACA). For Sumwalt the ACA is defined by altitude above ground level, an area history shows to be of high risk for aircraft accidents mainly due to workload and distractions (and, to no small amount, proximity to the ground). I expand on the concept of ACAs to include the last 1000 feet of climb or descent before leveling off -- also an area of high workload and distraction and potentially hazardous if the target altitude is missed, especially in descent. Why add these level-off transitions to the ACA? Because these are the times to employ some ACA-like techniques:
- Sterile cockpit: When within 1000 feet of a desired or assigned altitude, avoid nonessential communication. Don't use this time to talk about baseball with your passengers, or to call the FBO to confirm your rental car is standing by. Focus your concentration on making a safe and accurate level-off. Note: This may require briefing passengers before the flight that there will be times when you'll be busy and ask they don't both you. Suggestion: Use the pilot isolation feature of your intercom in the ACA if passengers are a distraction.
- Defer tasks: In the level-off ACA defer as much as possible until you've completed the transition to level flight, have the plane trimmed and all flow-check and checklist actions are complete. (More on checklists and flows in a future Leading Edge column.) Don't try to brief an approach or step through engine-temperature indications at the same time you're trying to level off. If it's not a requirement of the transition itself (for instance, changing pitch attitude and trim, or cowl flap position) or absolutely time-sensitive (like an ATC communication), it can wait. In fact, even a frequency change can often be delayed a few seconds until you get trimmed on altitude. Suggestion: Use the word "standby" if a controller calls with a re-route or other action requirement just as you're leveling from climb or descent.
- Focus on the task at hand: Employing sterile cockpit techniques and deferring non-essential tasks will help you concentrate on what matters most at this time: safely, accurately and smoothly transitioning to level flight at your new altitude.
Leveling From ClimbAccurately and smoothly leveling off from a climb comes through a series of actions that begin well before the desired altitude. I teach beginning the process of level-off at 1000 feet below your altitude target.
- 1000 feet to go
- Employ the techniques of the Altitude Critical Area.
- If your current rate of climb exceeds 500 feet per minute (fpm), lower pitch attitude to obtain a 500 fpm climb. This helps smooth the process of leveling off. It also puts you in the mental state of transitioning: If you think, "Climb to 7000 feet," you may get behind the airplane and bust altitude; instead, think, "Begin actions at 6000 feet to level off at 7000."
- Do not change power: This begins the process of acceleration toward cruise speed -- after all, you're trying to get to the destination.
- 100 feet to go
- If the airplane is equipped with cowl flaps -- and in accordance with manufacturers' recommendations -- close the cowl flaps. Some airplanes have a pitch change with cowl flap retraction. Leaving cowl flaps open as you level off increases drag and inhibits acceleration to cruise speed. Closing the cowl flaps a little before leveling off puts you in the mindset of making a change instead of blasting through your altitude.
- Adjust pitch attitude down slightly to smoothly transition toward your desired altitude.
- Reaching altitude
- Adjust pitch to level flight attitude.
- Keep climb power set; the airplane will accelerate to cruise speed more rapidly.
- After reaching target cruise speed, adjust power as desired.
- Complete a cockpit flow check and the cruise checklist as time permits.
Trim ChangeWhen leveling off from climb to cruise, in many airplanes you'll experience a dramatic change in trim. A stable airplane will attempt to maintain the indicated airspeed for which it is trimmed. If you climb at 110 knots, for instance, and your cruise indicated airspeed will be 140 knots, in the transition between climb and level you'll need to make a big nose-down change in trim or the airplane, trying to maintain trimmed speed, will want to continue to climb. The trim change is not immediate, either. Give forward pressure on the yoke and trim off that pressure. The airplane accelerates and "passes" that intermediate trimmed speed. Hold more pressure and re-trim. It may take several minutes before the airplane reaches its final speed, during which it will not be precisely trimmed at any time. Power changes may make this even more dynamic. You can see why concentration and deferral of tasks is important. You'll also find the trim change required at any given point is less if you start the level-off process at 1000 feet below altitude -- you begin the acceleration, and the trim change, sooner and arrive on altitude at something closer to cruise indicated airspeed.
On The StepThere's an archaic notion that airplanes, like speedboats, can get "on the step," or in a low-drag configuration that results in higher cruise speeds, by climbing slightly above altitude and then diving down in the transition. Although this may get the airplane up to cruise speed a little faster, this myth has been pretty much debunked and the final cruise speed will be the same. A common older practice of reducing power just as you level off (instead of accelerating under climb power, then pulling back) may have contributed to this notion. Another "step" to accelerating to cruise speed that may have more validity, however, involves controllable-pitch propellers ... especially three-bladed props. Although higher prop speeds generally generate more thrust, they also create more drag than the same prop at a lower rpm. In some airplanes this may be noticeable. For instance, an A36 Bonanza with a three-bladed propeller will accelerate smartly on level-off through about 130 knots indicated airspeed, but then the rate of airspeed increase noticeably slows. I think the propeller is creating a "drag wall" through which it takes more time to accelerate. I have found that, if I reduce rpm as the airplane is reaching 130 KIAS, the rate of acceleration continues unhindered up to cruise speed. In fact, if I want to cruise at a high rpm, I can pull the prop back a couple hundred rpm at 130 KIAS, accelerate to near final speed, and then advance the propeller again to sneak up on those last few knots. This is likely very airplane-specific, so the prop drag may not be evident in the airplane you fly.
Leveling From DescentIt's even more important to avoid descending through an altitude; after all, down is where the ground is. As with climbs, I teach a descent level-off technique that begins 1000 feet above my desired altitude.
- 1000 feet to go
- Adjust power and pitch attitude to transition to a 500 fpm descent.
- Employ Altitude Critical Area techniques.
- 100 feet to go
- Adjust pitch to smoothly transition toward level flight.
- Adjust pitch to level flight attitude.
- Adjust power and configuration as necessary for airspeed control.
- Complete a cockpit flow and applicable checklists as time permits.
Letting "George" Do ItThe Aviation Safety Reporting System notes it receives "several thousand" reports of altitude deviations each year. The vast majority of these "NASA reports" come from professional flight crews flying airline and corporate equipment -- which are flown on autopilot much of the time. The gist of these reports is that "altitude busts" come in three forms:
- Improper programming input
- Lack of monitoring
- Mechanical failure
Level-HeadedLeveling off, whether from climb or descent, by hand or using the autopilot, is a dynamic change in attitude, trim and performance. During the transition the airplane will naturally be out of precise trim, with a tendency to "bust" altitude without active intervention. Being "behind the airplane" makes altitude deviation even more likely, and invites jerky actions that are imprecise and uncomfortable to passengers. Automation carries an inherent risk of operator or mechanic failure that demands close monitoring by the pilot. There will be times when you need to modify your technique to fit traffic or terrain demands. But when you have the option, an SOP that begins the level-off transition 1000 feet early and includes specific actions at 100 feet to go and when reaching target altitude will help you make safer, smoother, more accurate and better-performing level offs.
Thomas Turner's Leading Edge columns are collected here.